var VSHADER_SOURCE =
    `attribute vec4 a_Position;
    attribute vec2 a_TexCoord;
    varying vec2 v_TexCoord;
    void main(){
        gl_Position=a_Position;
        v_TexCoord=a_TexCoord;
    }`;

// Fragment shader program
var FSHADER_SOURCE =
    `precision mediump float;
    uniform sampler2D u_Sampler0;
    uniform sampler2D u_Sampler1;
    varying vec2 v_TexCoord;
    void main(){
        vec4 color0=texture2D(u_Sampler0,v_TexCoord);
        vec4 color1=texture2D(u_Sampler1,v_TexCoord);
        gl_FragColor=color0*color1;
    }`;

function main() {
    // Retrieve <canvas> element
    var canvas = document.getElementById('webgl');

    // Get the rendering context for WebGL
    var gl = getWebGLContext(canvas);
    if (!gl) {
        console.log('Failed to get the rendering context for WebGL');
        return;
    }

    // Initialize shaders
    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log('Failed to intialize shaders.');
        return;
    }

    // 
    var n = initVertexBuffers(gl);
    if (n < 0) {
        console.log('Failed to set the vertex information');
        return;
    }

    if (!initTextures(gl, n)) {
        console.log('Failed to init textures');
        return;
    }

    // Specify the color for clearing <canvas>
    gl.clearColor(0.0, 0.0, 0.0, 1.0);

    // Clear <canvas>
    gl.clear(gl.COLOR_BUFFER_BIT);
}

function initVertexBuffers(gl) {
    var verticesTexCoords = new Float32Array([-0.5, 0.5, 0.0, 1.0, -0.5, -0.5, 0.0, 0.0,
        0.5, 0.5, 1.0, 1.0,
        0.5, -0.5, 1.0, 0.0,
    ]);

    var n = 4; // The number of vertices

    // Create a buffer object
    var vertexTexCoordBuffer = gl.createBuffer();
    if (!vertexTexCoordBuffer) {
        console.log('Failed to create the buffer object');
        return false;
    }

    // Write the vertex coordinates and colors to the buffer object
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexTexCoordBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, verticesTexCoords, gl.STATIC_DRAW);

    var FSIZE = verticesTexCoords.BYTES_PER_ELEMENT;
    //Get the storage location of a_Position, assign and enable buffer
    var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    if (a_Position < 0) {
        console.log('Failed to get the storage location of a_Position');
        return -1;
    }
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 4, 0);
    gl.enableVertexAttribArray(a_Position); // Enable the assignment of the buffer object

    // Get the storage location of a_Position, assign buffer and enable
    var a_TexCoord = gl.getAttribLocation(gl.program, 'a_TexCoord');
    if (a_TexCoord < 0) {
        console.log('Failed to get the storage location of a_TexCoord');
        return -1;
    }
    gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, FSIZE * 4, FSIZE * 2);
    gl.enableVertexAttribArray(a_TexCoord); // Enable the assignment of the buffer object

    return n;
}

function initTextures(gl, n) {
    var texture0 = gl.createTexture();
    var texture1 = gl.createTexture();
    var u_Sampler0 = gl.getUniformLocation(gl.program, 'u_Sampler0');
    if (u_Sampler0 < 0) {
        console.log('Failed to get the storget location of u_Sampler0');
        return false;
    }
    var u_Sampler1 = gl.getUniformLocation(gl.program, 'u_Sampler1');
    if (u_Sampler1 < 0) {
        console.log('Failed to get the storget location of u_Sampler1');
        return false;
    }
    var image0 = new Image();
    image0.onload = function() {
        loadTexture(gl, n, texture0, u_Sampler0, image0, 0);
    };
    image0.src = '../resources/redflower.jpg';
    var image1 = new Image();
    image1.onload = function() {
        loadTexture(gl, n, texture1, u_Sampler1, image1, 1);
    };
    image1.src = '../resources/circle.gif';
    return true;
}
var g_texUnit0 = false;
var g_texUnit1 = false;

function loadTexture(gl, n, texture, u_Sampler, image, texUnit) {
    gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);
    if (texUnit == 0) {
        gl.activeTexture(gl.TEXTURE0);
        g_texUnit0 = true;
    } else {
        gl.activeTexture(gl.TEXTURE1);
        g_texUnit1 = true;
    }
    gl.bindTexture(gl.TEXTURE_2D, texture);

    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, image);
    gl.uniform1i(u_Sampler, texUnit);
    if (g_texUnit0 && g_texUnit1) {
        gl.clear(gl.COLOR_BUFFER_BIT);
        gl.drawArrays(gl.TRIANGLE_STRIP, 0, n);
    }
}